{{Short description|Dictionary-based compression algorithm}} {{For|the Partido Socialdemocracia Chilena, also known by the initials SDCH|Chilean Social Democracy Party}} {{More citations needed|date=July 2016}} '''SDCH''' ('''Shared Dictionary Compression for HTTP'''), pronounced "sandwich", is a data compression algorithm created by Google, based on VCDIFF (RFC 3284).
SDCH achieves its effectiveness by using pre-negotiated dictionaries to "warm up" its internal state prior to encoding or decoding. These may either be already stored locally, or uploaded from a source and then cached.
It was supported natively in Google Chrome, Chromium, and Android, as well as on Google websites.<ref>{{cite web|url=https://lists.w3.org/Archives/Public/ietf-http-wg/2008JulSep/att-0441/Shared_Dictionary_Compression_over_HTTP.pdf|title=A Proposal for Shared Dictionary Compression Over HTTP|publisher=Google|last1=Butler|first1=Jon|author2=Wei-Hsin Lee|last3=McQuade|first3=Bryan|last4=Mixter|first4=Kenneth}}</ref><ref>{{cite web|url=https://groups.google.com/forum/#!forum/SDCH|title=SDCH Mailing List|publisher=Google Groups}}</ref>
SDCH compression was removed from Google Chrome, and other Chromium products, in version 59 (2017-06-05).<ref>{{cite web|url=https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/nQl0ORHy7sw|title=Intent to Unship: SDCH|accessdate=2017-05-02}}</ref>
Due to the diffing results and the data being compressed with the same coding, SDCH dictionaries aged relatively quickly and compression density became quickly worse than with the usual non-dictionary compression such as GZip. This created extra effort in production to keep the dictionaries fresh and reduced its applicability. Modern dictionary coding such as Shared Brotli has a more effective solution for this that fixes the dictionary aging problem.
==See also== *HTTP compression *SPDY
==References== {{Reflist}}
{{Internet-stub}} {{Algorithm-stub}}
Category:Data compression